1. Field of the Invention
The present invention relates to a laser tracking interferometer for measuring the displacement and position of a moving object with high accuracy while tracking the moving object. More particularly, the present invention relates to an improvement of a laser tracking interferometer which directs a laser beam to a retroreflector to sense a displacement of the retroreflector using the interference with a laser beam back reflected from the retroreflector as well as to track the retroreflector using a change in position of the optical axis of the laser beam.
2. Description of the Related Art
Such a laser tracking interferometer is known which directs a laser beam to a retroreflector to sense a displacement of the retroreflector using the interference with a laser beam back reflected from the retroreflector as well as to track the retroreflector using a change in position of the optical axis of the laser beam.
For example, as shown in FIG. 1 (overall configuration) and FIG. 2 (the detailed configuration of a laser interferometer), disclosed in the description of US Pat. No. 6,147,748 (hereinafter referred to as Patent Document 1) is a laser tracking interferometer which is designed to measure a relative displacement between a retroreflector 10 and the center C of a reference sphere 12.
This laser interferometer includes the reference sphere 12; a carriage 14 designed to rotate about the center C of the reference sphere 12; a laser interferometer 16 including a light source (not shown); and a condenser lens 18 for focusing a measurement beam at the center C or on the surface of the reference sphere 12. The laser interferometer 16 and the condenser lens 18 are provided on the carriage 14.
In this arrangement, as shown in FIG. 2, a light beam emitted from a light source enters a non-polarizing beam splitter (NPBS) 24 through an optical fiber 20 and a collimator lens 22, and a portion of the light beam is reflected off the NPBS 24 to be directed as an optical reference counter signal to a photodetector (not shown) through a polarizing plate 26 and an optical fiber 28. The optical reference counter signal is used to compensate polarization mixing in the optical fiber. On the other hand, the light beam that has passed through the NPBS 24 is separated at a polarizing beam splitter (PBS) 30 into two. One goes straight through as a reference and interfered with a measurement beam for obtaining measurement signal. The other is reflected off the PBS 30 and then transmitted as the measurement beam to the center C or the surface of the reference sphere 12 through a quarter-wave (λ/4) plate 32 and the condenser lens 18.
The measurement beam that has been reflected off the surface of the reference sphere 12 is directed to the retroreflector 10 through the condenser lens 18, the λ/4 plate 32, the PBS 30, a λ/4 plate 34, and a NPBS 36.
The measurement beam that has been reflected off the retroreflector 10 reenters the laser interferometer 16. A portion of the measurement beam that is incident upon the laser interferometer 16 is reflected off the NPBS 36 to enter a position sensitive detector (PSD) 38. On the other hand, the remaining portion passes through the λ/4 plate 34, the PBS 30, a polarizing plate 40, and an optical fiber 42 to interfere with the reference beam and then enter the photodetector.
Since the output from the photodetector varies according to the interference pattern of the incident interference beam, the output from the photodetector can be used to measure the displacement of the retroreflector 10 with respect to the center C of the reference sphere 12.
The reference sphere 12 having a high sphericity allows the distances from the center C of the reference sphere 12 to the surface to be constant with high accuracy. This makes it possible to measure the displacement of the retroreflector 10 with high accuracy with respect to the center C of the reference sphere 12 even when the laser interferometer 16 tracks the retroreflector 10 to rotate about the center C of the reference sphere 12.
On the other hand, the retroreflector 10 is tracked as follows. That is, a portion of the measurement beam incident upon the laser interferometer 16 enters the PSD 38. Thus, the position of the carriage 14 can be controlled to allow the position of the measurement beam on the PSD 38 to be kept constant all the time, thereby automatically tracking the retroreflector 10. This is because the positional displacement of the measurement beam incident upon the PSD 38 varies depending on the displacement of the retroreflector 10 in a direction perpendicular to the optical axis of the measurement beam, so that a displacement of the retroreflector 10 in a direction perpendicular to the optical axis of the measurement beam causes the optical axis of the measurement beam back reflected from the retroreflector 10 to be displaced in parallel.
On the other hand, as shown in FIG. 3, a laser tracking interferometer for measuring the relative displacement between a retroreflector 106 serving as a reference and a retroreflector 110 serving as a target is disclosed in the Publication of Japanese Patent No. 2603429 (hereinafter referred to as Patent Document 2).
This laser interferometer includes: a first retroreflector 106 disposed at a fixed location; a second retroreflector 110 disposed on a moving object; a rotating portion 108 which is rotatable about each of the X-axis and the Y-axis which orthogonally intersect each other at the center of the first retroreflector 106; means for directing a laser beam produced by lasing in a laser light source (not shown) to the rotating portion 108 irrespective of the rotation of the rotating portion 108; and an optical system made up of a plurality of optical components (λ/4 plates 148 and 164, prisms 150, 152, 158, 160, and 162, and a PBS 154) which are each disposed at a fixed location in the rotating portion 108.
This optical system is designed such that a laser beam directed to the rotating portion 108 is divided at the PBS 154 into two, one of which goes through an optical path orthogonal to the X-axis and enters the first retroreflector 106 while the other laser beam further travels along the optical path and enters the second retroreflector 110, thereby allowing for providing the reflected beams from each of the first and second retroreflectors 106 and 110.
Also included are a detecting portion (not shown) for detecting the amount of movement of the second retroreflector 110 based on the interference between the two reflected beams obtained via the optical system; a quadrant photodiode (QPD) 112 serving as position sensitive detector, which is disposed at a fixed location in the rotating portion 108 and upon which a portion of the reflected beam from the second retroreflector 110 is incident, for providing a position signal corresponding to the amount of deviation of the laser beam incident upon the second retroreflector 110; and control means (not shown) for controlling the rotational position of the rotating portion 108 about the X-axis and the Y-axis so that the amount of deviation becomes zero, based on the position signal from the position sensitive detector.
On the other hand, a laser tracking length-measuring interferometer which is provided with a tracking mirror in an optical path of an interference optical system instead of the reference sphere is disclosed in Japanese Patent Laid-Open Publication No. 2002-98510 (hereinafter referred to as Patent Document 3). With this interferometer, a laser beam is directed to the center of the reflection plane of the tracking mirror, and the tracking mirror can be controlled for the reflected beam to be directed in a desired direction, thereby allowing the reflected beam to impinge upon a retroreflector serving as an object to be measured for tracking purposes.
However, the technique described in Patent Document 1 has the following problems: (1) when the measurement beam is condensed so as to focus at the center C of the reference sphere 12, this arrangement is susceptible to runout of the rotational mechanism (a deviation between the actual locus, which a point on the rotational mechanism draws when the rotational mechanism rotates, and the ideal locus). That is, a deviation of the center of the focus of the measurement beam from the position C due to the runout of the rotational mechanism causes the S/N of a signal produced by a photodetector to degrade, thereby disabling the measurement of displacement. This arrangement is thus susceptible to the runout of the rotational mechanism. On the other hand, there is also another problem with this technique: (2) with a focus on the surface of the reference sphere 12, this arrangement is susceptible to flaws and dust particles on the surface of the reference sphere, and particularly more susceptible to small flaws and dust particles when the focus has a smaller diameter.
According to the technique described in Patent Document 2, when a metal sphere or a glass sphere coated with metal is employed as a reference sphere, the technique also has the following problems as with the technique disclosed in Patent Document 1. That is, (1) with the center of the reference sphere employed as the focus, the arrangement is not robust to the runout of the rotational mechanism; and (2) with the surface of the reference sphere employed as the focus, the arrangement is susceptible to flaws and dust particles on the surface of the reference sphere.
Furthermore, when a sphere formed of a material having a refractive index of 2.0 is employed as a reference sphere, the arrangement has a problem, in addition to the aforementioned problem (2), that such a sphere is not commercially available in general, and thus the reference sphere is expensive and difficult to obtain.
On the other hand, with the technique disclosed in Patent Document 3, when the center of a laser beam is not aligned with the center of rotation of a mirror, this error may cause an error in the measurement of length. However, the center position of the laser beam is difficult to measure with high accuracy, and thus difficult to be aligned with the center of rotation of the mirror with high accuracy. Furthermore, a steel sphere and a semisphere are seated under pressure by means of tensile force exerted by a helical spring, thereby causing an increase in friction between the steel sphere in a three-point spherical seat and the semispherical portion of the mirror. It is thus difficult to provide control with accuracy. Additionally, although a high-precision sphere can be made relatively easily, a high-precision semisphere is expensive. Thus, there is a problem that the semispherical mirror is made only at high costs.